The 'Wrecking Ball' hitmaker was rushed to a US medical centre after having a severe allergic reaction to antibiotics prescribed to her for a fever.
The 21-year-old star was forced to cancel her performance at the Sprint Centre in Kansas City, Missouri, as part of her 'Bangerz' world tour, and a press release from the venue stated she had been "placed on medical rest by her doctors."
Another source told E! News the singer had "an absolutely horrible reaction" to medication.
Miley also took to Twitter from her hospital bed to apologise for the cancellation.
She tweeted: "Kansas I promise I'm as heartbroken as you are. I wanted to badly 2 b there 2night.
"Not being with yall makes me feel s***tier than I already do (sic)."
The 'Adore You' singer later posted a picture of herself in a hospital gown, with a toy octopus covering her face, writing: "Mr. Octopussy & some amazing Drs are taking good care of me (sic)."
Miley first revealed she was not feeling well on Monday (14.04.14), tweeting: "I woke up like disssss #SICK Fever fever go away (sic)."
News of her medical scare comes after she was devastated by the sudden death of her dog, Floyd, on April 1 and admitted she was "emotionally and physically exhausted."
She also cancelled another show in Charlotte, North Carolina last week when she came down with the flu.